Waterjets are known for providing marine surface vessel propulsion. Waterjets provide more efficient operation in comparison to propellers at relatively high speeds, such as between 50 and 100 kilometers per hour. Waterjets operate by drawing water through a duct from an opening located below the waterline of the vessel, and exhausting a high speed jet of water at the rear of the vessel above the waterline. The reactive force of the jet of water provides a propulsive force.
In prior arrangements, the duct includes an intake arrangement comprising an opening which is flush with the submerged surface of the hull. This opening is the termination of an inclined duct leading upwardly to a pump impeller, which raises the water pressure and speed for discharge through the exhaust above the vessel's water line.
At low vessel speeds, water enters the intake from upstream, downstream and sideways directions. Generally, the upstream direction corresponds to the direction of travel of the water vessel, and the downstream direction corresponds to the opposite direction. At higher speeds the water entry is predominantly from the upstream direction due to the momentum of the approaching water in the reference frame of the vessel.
In order to provide greatest efficiency, a relatively larger intake aperture area is optimal at lower speeds in comparison to the optimal intake aperture area at higher speeds. A relatively small intake aperture area which is optimal for relatively high speed may lead to excessive turning of water around the lip at the downstream edge of the opening at low speed, leading to cavitation on an inner surface of the lip, within the duct. On the other hand, a relatively large intake aperture area which is optimal for relatively low speed may lead to excess water entering the duct at high speed, leading to cavitation on an outer surface of the lip, i.e. on the underneath of the hull. Such cavitation leads to reduced efficiency, and may in extreme cases lead to damage or reduced service life of the impellor, particularly where the cavitation occurs inside the duct.
Conventional intake designs therefore have an area which provides a compromise between low and high speed operation.
One solution to this problem is to provide a variable geometry intake. For example, U.S. Pat. No. 3,942,463 discloses a system comprising an intake duct having a flexible upper surface. The upper surface is actuated using bell cranks and pusher bars.
JP2000128080 discloses a system with movable sidewalls which can widen or narrow the aperture according to vessel speeds.
JP7101392 discloses a system with upper and lower parallel water jet ducts, with an obliquely frontward downward extendible and retractable moving scoop member to increase flow into the lower duct only at low vessel speeds.
However, each of these variable geometry systems adds further complexity and weight, and is vulnerable to foreign object damage, particularly at high speeds.
Relative positional terms such as “upstream”, “downstream”, “above” and “below” will be understood as referring to the in use orientation of the water vessel.
The present invention provides a duct arrangement for a waterjet that seeks to address some or all of the aforementioned problems.